Systems of this kind are used, for example for checking persons wishing access to locations, for the identification of vehicles, for the identification of merchandise or, more generally, for identifying any badge or other portable device intended for recognition of its bearer.
Generally speaking, the system comprises a base station which is coupled to the data carrier, for example a badge. The latter includes a memory, possibly a programmable memory, which stores notably an identifier which enables identification of the person, animal or object wearing the badge. During use the badge is interrogated by the base station by application of an interrogation signal. In response thereto the badge supplies a return signal which contains the identifier stored in the badge. The base station demodulates and decodes the return signal and, if the badge is recognized and accepted, the base station validates the recognition and takes the appropriate actions.
However, these systems are often disrupted when two or more badges must be simultaneously recognized. Actually, it may be difficult for a base station to decode signals simultaneously arriving from two badges. Some systems solve this problem by imposing exchange protocols based on sequences of interrogations and responses. However, this complicates the hardware means used and the processing time is also considerably prolonged. The document GB 2 157 132 A describes an identification system which includes an interrogation station capable of operating simultaneously with a plurality of badges to be identified. This system suffers from the described drawbacks. Actually, because of such a protocol the badge must be provided with specific means in order to establish this protocol, so that the structure of the badge is significantly complicated and its manufacturing cost increased. Moreover, error sources occur when the transmission circumstances are such that the signals exchanged are drowned in noise.